When I added about 600 lbs. to the back of my 35' fifth wheel, about 140 lbs. came off the pin. Not a big deal in my case and the trailer actually rode better when I had the weight back there. You're already pretty light insofar as the pin weight is concerned so your results may vary.
But do be aware that what is hanging off the back is really going to move around...a lot. Double check securing straps and the reliability of all attachments.
After rereading your first post, you don't really know what your pin weight is so I would weigh before and after adding the weight on the back. You may not be happy with the results, and it's easier to change plans before you leave rather than on the road somewhere.
